Hydrogen donor solvent processes for use in the hydrogenation and liquefaction of coal are of particular interest among known coal conversion processes for the production of useful petroleum-like liquids, i.e., 1000.degree. F.- liquid products. In such processes, crushed coal is contacted at elevated temperature and pressure with a solvent, often a liquid fraction derived from within the process, which acts as a hydrogen transfer agent or donor. The solvent supplies hydrogen to the hydrogen-deficient coal molecules, as molecules are thermally cracked and cleaved from the disintegrating coal solids.
Coal is largely comprised of polymerized multi-ring aromatic structures, and in the breaking coal molecules each bond rupture results in the formation of extremely reactive free radicals. These moities, when early stabilized by the addition of a hydrogen atom, if sufficiently small, may be evolved as a portion of the desired petroleum-like liquid product. If the moities become excessively large before they are stabilized, undesirable 1000.degree. F.+ liquid products can form. Also, the moities can form polymeric products, and the fragments may remain with, or form a part of the char or coke that is produced. Sufficient hydrogen must be available, and effectively utilized to avoid repolymerization of the moities to form char or choke.
Coal, of course, is not a pure hydrocarbon. It contains volatile matter, fusain, mineral matter and sulfur, much as pyritic sulfur, inorganic sulfates and organic sulfur compounds. Coal also contains bitumin and humin which have large, flat, aromatic, lamellar structures that differ in molecular weight, degree of aromaticity, oxygen and nitrogen contents and degree of cross-linking. The product liquids produced from coal thus vary widely in composition. Whereas much of the coal has been successfully converted to useful petroleum-like liquids, the amount of such liquids which can be produced is quite variable. The liquid products themselves also vary considerably in composition, and liquids are only a portion of the total products that are produced. The product liquids contain fusinite and ash, as well as char and sludge, which must be separated from the liquids. The heavy products from such coal liquefaction processes, characterized as "liquefaction bottoms" and consisting of 1000.degree. F.+ organics, ash and carbon residue (fusinite), consist largely of carbon, 60-70 weight percent, and about 20 weight percent ash. The liquefaction bottoms, which are less useful than the 1000.degree. F.- liquids, generally contain 45-55 weight percent of the original feed coal to the process.
Various attempts have been made to convert more of the carbon of the coal to useful liquid products. It is thus desirable to obtain higher levels of conversion, and to reduce the level of formation of the excessively high molecular weight hydrocarbons which occur in the process. One approach to improving carbon efficiency is described, e.g., in U.S. Pat. No. 3,700,583 issued to Salamony et al on Oct. 24, 1972. This process describes the use of quinones, particularly quinone derivatives of mono- and/or polynuclear aromatic compounds, certain halogens and halogen halides thereof as carbon-radical scavengers which are added with the hydrogen donor solvent to the coal liquefaction zone to increase the amount of low molecular weight hydrocarbons which are formed within the liquid product, as measured by an increase in the total amount of benzene-soluble liquids in the product.
Higher levels of conversion have also been obtained by the use of polar solvents added to the coal liquefaction zone as described in application Ser. Nos. 607,433 and 641,489, supra. In accordance with the processes described in the former application, a heterocyclic nitrogen compound, or mixture of heterocyclic nitrogen compounds, and in the latter application a heterocyclic oxygen or sulfur compound, or admixture of such compounds, is added to a donor solvent fraction indiginous to the process, and the donor solvent, containing the added polar-solvent in suitable concentration, is recycled to the coal liquefaction zone, thus increasing the conversion of coal to lower molecular weight, more useful petroleum-like liquid products than obtainable in a process otherwise similar except that no polar solvent was employed. It is believed that the polar compounds progressively enhance dispersion of the high molecular weight compounds, notably those boiling at 1000.degree. F.+, as liquefaction of the coal proceeds. The free radicals produced by the thermal cracking of the large coal molecules are thus known to be extremely short-lived, and are formed principally at the solid interfaces wherein the coal solids particles are being dissolved. By improving contact between the hydrogen donor solvent and these moities, repolymerization of some of these moities with other molecules or with each other is suppressed. The greater effectiveness of the hydrogen donor molecules in their role of reaching the extremely reactive-free radicals as they are formed, and more effectively hydrogenating said radicals is thus believed to account largely for these improvements.
Whereas processes utilizing the addition of polar solvents to the donor solvent offer advantages over prior art processes, such process has, nonetheless, been found susceptible of further improvement.
Among the objects of this invention are:
To provide a new and improved process wherein polar solvents are employed to provide further increased yields of the desirable 1000.degree. F.- petroleum-like liquid products with further decreased levels of coke or char.
To provide a new and improved hydrogen donor coal liquefaction process, particularly one which utilizes a polar solvent or compound to produce greater quantities of the more useful petroleum-like liquids, with decreased amounts of char and coke.